Twitch fibers of certain frog skeletal muscles (e.g., sartorius, ext. 1. dig. IV) possess multiple, polyneuronal innervation. In adult animals, the end-plates, which are uually located several millimeters apart, range in number from 2 to 5 per fiber. Sartorius twitch fibers differ significantly in their physical, biochemical, and contractile properties. Histochemical techniques for the demonstration of acetylcholinesterase (AChE) will be used to test the possibility that the pattern of innervation (number and spacing of end-plates) in adult muscle fibers is correlated with the properties of individual fibers. In addition, intracellular recording of synaptic events, AChE staining, and alpha-bungarotoxin binding will be used to examine how the pattern of innervation and synaptic efficacy changes during development from late metamorphosis through adulthood. Preliminary results suggest that in adult muscle fibers, the synaptic effficacy at junctions on a common muscle fiber innervated by different parent axons is similar. Electrophysiological techniques and AChE staining will be used to examine the possibility that a trophic infuence from muscles to motoneurons may regulate transmitter release. The proposed experiments are expected to provide information about the factors which determine the number, location, and efficacy of synapses in amphibian twitch muscle fibers.